Semiconductor device, light-emitting device, and electronic device

ABSTRACT

An object is to prevent an operation defect and to reduce an influence of fluctuation in threshold voltage of a field-effect transistor. A field-effect transistor, a switch, and a capacitor are provided. The field-effect transistor includes a first gate and a second gate which overlap with each other with a channel formation region therebetween, and the threshold voltage of the field-effect transistor varies depending on the potential of the second gate. The switch has a function of determining whether electrical connection between one of a source and a drain of the field-effect transistor and the second gate of the field-effect transistor is established. The capacitor has a function of holding a voltage between the second gate of the field-effect transistor and the other of the source and the drain of the field-effect transistor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.17/172,213, filed Feb. 10, 2021, now allowed, which is a continuation ofU.S. application Ser. No. 16/834,308, filed Mar. 30, 2020, now U.S. Pat.No. 10,950,633, which is a continuation of U.S. application Ser. No.16/026,115, filed Jul. 3, 2018, now U.S. Pat. No. 10,622,380, which is acontinuation of U.S. application Ser. No. 15/341,069, filed Nov. 2,2016, now U.S. Pat. No. 10,032,798, which is a continuation of U.S.application Ser. No. 14/640,235, filed Mar. 6, 2015, now U.S. Pat. No.9,508,709, which is a continuation of U.S. application Ser. No.13/612,073, filed Sep. 12, 2012, now U.S. Pat. No. 8,975,709, whichclaims the benefit of a foreign priority application filed in Japan asSerial No. 2011-202690 on Sep. 16, 2011, all of which are incorporatedby reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

One embodiment of the present invention relates to a semiconductordevice. Further, one embodiment of the present invention relates to alight-emitting device. Furthermore, one embodiment of the presentinvention relates to an electronic device.

2. Description of the Related Art

In recent years, development of semiconductor devices includingfield-effect transistors has been advanced.

As an example of the semiconductor devices, there is a semiconductordevice performing a desired operation by controlling the amount ofcurrent flowing between a source and a drain of the field-effecttransistor (for example, see Patent Document 1).

REFERENCE [Patent Document]

-   [Patent Document 1] Japanese Published Patent Application No.    2008-083085

SUMMARY OF THE INVENTION

However, conventional semiconductor devices have a problem in that theamount of current flowing between a source and a drain is difficult tocontrol due to fluctuation in threshold voltage of a field-effecttransistor. When the amount of current flowing between the source andthe drain cannot be controlled, for example, an operation defect occursin the semiconductor device.

An object of one embodiment of the present invention is to prevent anoperation defect and/or to reduce an influence of fluctuation inthreshold voltage of a field-effect transistor.

In one embodiment of the present invention, a field-effect transistorincluding a first gate and a second gate which overlap with each otherwith a channel formation region therebetween is used. By controlling thepotential of the second gate, the threshold voltage of the field-effecttransistor is determined. With the above structure, the amount ofcurrent flowing between a source and a drain of the field-effecttransistor in operation can be controlled.

One embodiment of the present invention is a semiconductor deviceincluding a field-effect transistor, a switch, and a capacitor.

The field-effect transistor includes a first gate and a second gatewhich overlap with each other with a channel formation regiontherebetween. The threshold voltage of the field-effect transistorvaries depending on the potential of the second gate. The field-effecttransistor may be a normally-on transistor. For example, thefield-effect transistor may be a depletion transistor.

The switch has a function of determining whether electrical connectionbetween one of a source and a drain of the field-effect transistor andthe second gate of the field-effect transistor is established.

The capacitor has a function of holding a voltage between the secondgate of the field-effect transistor and the other of the source and thedrain of the field-effect transistor.

According to one embodiment of the present invention, an effect ofpreventing an operation defect and/or an effect of reducing an influenceof fluctuation in threshold voltage of a field-effect transistor can beobtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B-1, 1B-2, and 1B-3 illustrate an example of a semiconductordevice.

FIGS. 2A to 2C each illustrate an example of a light-emitting device.

FIGS. 3A to 3C each illustrate an example of a light-emitting device.

FIGS. 4A to 4C each illustrate an example of a light-emitting device.

FIGS. 5A and 5B illustrate an example of a light-emitting device.

FIGS. 6A and 6B illustrate an example of a light-emitting device.

FIG. 7 illustrates an example of a light-emitting device.

FIGS. 8A and 8B each illustrate an example of a field-effect transistor.

FIGS. 9A to 9C illustrate a structure example of an active matrixsubstrate.

FIG. 10 illustrates a structure example of a light-emitting device.

FIGS. 11A to 11C each illustrate an electronic device.

FIGS. 12A and 12B illustrate an example of an electronic device.

DETAILED DESCRIPTION OF THE INVENTION

Embodiment according to the present invention will be described below.Note that it will be readily appreciated by those skilled in the artthat details of the embodiments can be modified in various ways withoutdeparting from the spirit and scope of the present invention. Thus, thepresent invention should not be limited to the description of thefollowing embodiments.

Note that part of or all the contents (for example, the content shown inthe specification or the drawings) in one embodiment can be combinedwith part of or all the contents in any of the other embodiments asappropriate. In addition, part of the contents in one embodiment can bereplaced with part of the contents in any of the other embodiments asappropriate.

Further, the ordinal numbers such as “first” and “second” are used toavoid confusion between components and do not limit the number ofcomponents.

Embodiment 1

In this embodiment, an example of a semiconductor device including afield-effect transistor having two gates will be described withreference to FIGS. 1A, 1B-1, 1B-2, and 1B-3 .

A semiconductor device in FIG. 1A includes a field-effect transistor Tr,a switch Sw, and a capacitor Cp.

The field-effect transistor Tr includes a first gate and a second gate.In the field-effect transistor Tr, the first gate and the second gateoverlap with each other with a channel formation region therebetween.The threshold voltage of the field-effect transistor Tr depends on thepotential of the second gate.

As the field-effect transistor Tr, an enhancement or depletionfield-effect transistor can be used.

The switch Sw has a function of determining whether electricalconnection between one of a source and a drain of the field-effecttransistor Tr and the second gate of the field-effect transistor Tr isestablished.

The capacitor Cp has a function of holding a voltage between the secondgate of the field-effect transistor Tr and the other of the source andthe drain of the field-effect transistor Tr.

Next, as an example of a method of driving the semiconductor device ofthis embodiment, a method of driving the semiconductor device in FIG. 1Awill be described with reference to FIGS. 1B-1 to 1B-3 . Note that here,a description is given of the case where the field-effect transistor Tris a depletion n-channel transistor, as one example.

In the method of driving the semiconductor device in FIG. 1A, the switchSw is turned on (brought into an on state) in a period T1 as illustratedin FIG. 1B-1 . The first gate of the field-effect transistor Tr issupplied with a potential V1. Further, the second gate of thefield-effect transistor Tr is supplied with a potential V2. Furthermore,the other of the source and the drain of the field-effect transistor Tris supplied with a potential Vb. Note that the value of V2 is largerthan the value of V1−Vb.

At this time, electrical connection between the second gate and thedrain of the field-effect transistor Tr is established so that thepotential of the second gate and that of the drain of the field-effecttransistor Tr each become the potential V2. Accordingly, depending onthe potential V2, the threshold voltage (also referred to as Vth) of thefield-effect transistor Tr is negatively shifted.

For example, when the threshold voltage of the field-effect transistorTr in an initial state is assumed to be Vth0, the threshold voltage ofthe field-effect transistor Tr in the period T1 is Vth0−ΔVth. In thiscase, the value of ΔVth depends on the value of the potential V2.Accordingly, the value of the threshold voltage of the field-effecttransistor Tr varies depending on the value of the potential V2.

A voltage between the first gate and the source (this voltage is alsoreferred to as Vgs) of the field-effect transistor Tr becomes V1−Vb. Atthis time, the value of V1−Vb is larger than the threshold voltage ofthe field-effect transistor Tr in the period T1. Accordingly, thefield-effect transistor Tr is turned on.

Next, in a period T2, the switch Sw is turned on. Further, the firstgate of the field-effect transistor Tr is supplied with the potentialV1. Furthermore, the second gate of the field-effect transistor Tr isbrought into a floating state.

At this time, the field-effect transistor Tr remains on. Accordingly,current flows between the source and the drain of the field-effecttransistor Tr, so that the potential of the second gate of thefield-effect transistor Tr is changed. As a result, the thresholdvoltage of the field-effect transistor Tr is positively shifted, and thefield-effect transistor Tr is turned off at the time when the thresholdvoltage of the field-effect transistor Tr becomes V1−Vb or higher. Inthis manner, the data of the threshold voltage of the field-effecttransistor Tr can be obtained.

Next, in a period T3, the switch Sw is turned off. Further, thepotential of the first gate of the field-effect transistor Tr is set toV1+Vsig, so that the first gate of the field-effect transistor Tr isbrought into a floating state. Note that Vsig represents the potentialof a data signal. Furthermore, the second gate of the field-effecttransistor Tr is brought into a floating state. The one of the sourceand the drain of the field-effect transistor Tr is supplied with apotential Va.

At this time, the field-effect transistor Tr is turned on, and currentflows between the source and the drain of the field-effect transistorTr. At this time, the potential of the other of the source and the drainof the field-effect transistor Tr is set to a potential Vc.

For example, in the case where the field-effect transistor Tr operatesin a saturation region, the value of current flowing between the sourceand the drain (Ids) of the field-effect transistor Tr depends on thevalue of the data signal input to the first gate, regardless of thethreshold voltage of the field-effect transistor Tr. Accordingly, forexample, in the case where Vgs is larger than V1−Vb, the field-effecttransistor Tr is turned on; thus, current flows between the source andthe drain.

Even in the case where the potential of the other of the source and thedrain of the field-effect transistor Tr is changed due to deteriorationof the field-effect transistor Tr or the like, a voltage between thefirst gate and the source of the field-effect transistor Tr can beprevented from being changed because the first gate and the second gateof the field-effect transistor Tr are in a floating state and thecapacitor Cp is provided.

Note that a mobility correction period may be provided between theperiod T2 and the period T3 and the potential of the second gate of thefield-effect transistor Tr may be set depending on the mobility of thefield-effect transistor Tr. Accordingly, an influence of fluctuation inmobility of the field-effect transistor Tr can be prevented.

The above is the description of the example of a method of driving thesemiconductor device in this embodiment.

As described with reference to FIGS. 1A, 1B-1, 1B-2, and 1B-3 , in anexample of the semiconductor device in this embodiment, a period duringwhich the data of the threshold voltage is obtained (e.g., the periodT2) is provided so that the data of the threshold voltage of thefield-effect transistor is obtained in advance. Accordingly, the amountof current flowing between the source and the drain of the field-effecttransistor can be determined regardless of the threshold voltage of thefield-effect transistor; thus, an influence of fluctuation in thresholdvoltage of the field-effect transistor can be prevented. Further, aninfluence of deterioration of the field-effect transistor can beprevented.

In an example of the semiconductor device in this embodiment, thefield-effect transistor including the first gate and the second gatewhich overlap with each other with the channel formation regiontherebetween is used. With such a structure, even in the case where thefield-effect transistor is a depletion transistor, the data of thethreshold voltage of the field-effect transistor can be obtained. Thereason of this is as follows: since the threshold voltage of thefield-effect transistor can be shifted in response to the potential ofthe second gate, the field-effect transistor can be off even when thefield-effect transistor is an n-channel transistor, the thresholdvoltage of the field-effect transistor in the initial state is anegative value and thus the field-effect transistor is a normally-ontransistor, and a voltage between the first gate and the source of thefield-effect transistor is not a negative value. Accordingly, the amountof current flowing between the source and the drain of the field-effecttransistor can be determined regardless of the threshold voltage of thefield-effect transistor; thus, an influence of fluctuation in thresholdvoltage of the field-effect transistor can be prevented.

As described above, in an example of the semiconductor device in thisembodiment, the amount of current flowing between the source and thedrain of the field-effect transistor can be controlled, and thus anoperation defect can be prevented.

Embodiment 2

In this embodiment, an example of a light-emitting device including afield-effect transistor having two gates will be described withreference to FIGS. 2A to 2C, FIGS. 3A to 3C, FIGS. 4A to 4C, FIGS. 5Aand 5B, and FIGS. 6A and 6B.

A light-emitting device in FIG. 2A includes wirings 151 to 158,field-effect transistors 111 to 118, capacitors 121 and 122, and alight-emitting element (also referred to as EL) 140.

The wiring 151 functions as a data signal line for supplying a datasignal or the like.

The wiring 152 functions as a potential supply line for supplying apotential or the like.

The wiring 153 functions as a gate signal line for supplying a gatesignal, which is a pulse signal, or the like.

The wiring 154 functions as a gate signal line for supplying a gatesignal, which is a pulse signal, or the like.

The wiring 155 functions as a gate signal line for supplying a gatesignal, which is a pulse signal, or the like.

The wiring 156 functions as a potential supply line for supplying apotential or the like.

The wiring 157 functions as a potential supply line for supplying apotential or the like.

The wiring 158 functions as a potential supply line for supplying apotential or the like.

One of a source and a drain of the field-effect transistor 111 iselectrically connected to the wiring 151. A gate of the field-effecttransistor 111 is electrically connected to the wiring 153.

One of a source and a drain of the field-effect transistor 112 iselectrically connected to the other of the source and the drain of thefield-effect transistor 111. A gate of the field-effect transistor 112is electrically connected to the wiring 154.

One of a pair of electrodes of the capacitor 121 is electricallyconnected to the other of the source and the drain of the field-effecttransistor 111.

The field-effect transistor 113 includes a first gate and a second gatewhich overlap with each other with a channel formation regiontherebetween. The first gate of the field-effect transistor 113 iselectrically connected to the other of the source and the drain of thefield-effect transistor 112.

One of a source and a drain of the field-effect transistor 114 iselectrically connected to one of a source and a drain of thefield-effect transistor 113. The other of the source and the drain ofthe field-effect transistor 114 is electrically connected to the secondgate of the field-effect transistor 113. A gate of the field-effecttransistor 114 is electrically connected to the wiring 153.

One of a pair of electrodes of the capacitor 122 is electricallyconnected to the second gate of the field-effect transistor 113. Theother of the pair of electrodes of the capacitor 122 is electricallyconnected to the other of the source and the drain of the field-effecttransistor 113.

One of a source and a drain of the field-effect transistor 115 iselectrically connected to the wiring 152. The other of the source andthe drain of the field-effect transistor 115 is electrically connectedto the one of the source and the drain of the field-effect transistor113. A gate of the field-effect transistor 115 is electrically connectedto the wiring 154.

One of a source and a drain of the field-effect transistor 116 iselectrically connected to the wiring 156. The other of the source andthe drain of the field-effect transistor 116 is electrically connectedto the first gate of the field-effect transistor 113. A gate of thefield-effect transistor 116 is electrically connected to the wiring 153.

One of a source and a drain of the field-effect transistor 117 iselectrically connected to the wiring 157. The other of the source andthe drain of the field-effect transistor 117 is electrically connectedto the other of the pair of electrodes of the capacitor 121 and theother of the pair of electrodes of the capacitor 122. A gate of thefield-effect transistor 117 is electrically connected to the wiring 153.

One of a source and a drain of the field-effect transistor 118 iselectrically connected to the wiring 158. The other of the source andthe drain of the field-effect transistor 118 is electrically connectedto the second gate of the field-effect transistor 113. A gate of thefield-effect transistor 118 is electrically connected to the wiring 155.

One of an anode and a cathode of the light-emitting element 140 iselectrically connected to the other of the source and the drain of thefield-effect transistor 113. As the light-emitting element 140, forexample, an electroluminescent element (also referred to as EL element)can be used.

A light-emitting device in FIG. 2B is different from the light-emittingdevice in FIG. 2A in a connection relation of the field-effecttransistor 113 and a connection relation of the field-effect transistor117.

In the light-emitting device in FIG. 2B, the other of the source and thedrain of the field-effect transistor 113 is electrically connected tothe other of the source and the drain of the field-effect transistor112. The first gate of the field-effect transistor 113 is electricallyconnected to the other of the pair of electrodes of the capacitor 121.Furthermore, the other of the source and the drain of the field-effecttransistor 117 is electrically connected to the other of the source andthe drain of the field-effect transistor 112 and the other of the pairof electrodes of the capacitor 122.

A connection relation of the field-effect transistor 116 in thelight-emitting device in FIG. 2C is different from that in thelight-emitting device in FIG. 2B. Further, the wiring 156 is notprovided in the light-emitting device in FIG. 2C unlike in thelight-emitting device in FIG. 2A.

In the light-emitting device in FIG. 2C, the one of the source and thedrain of the field-effect transistor 116 is electrically connected tothe first gate of the field-effect transistor 113. The other of thesource and the drain of the field-effect transistor 116 is electricallyconnected to the other of the pair of electrodes of the capacitor 122.Note that the field-effect transistor 113 may be an enhancementtransistor.

With the structure in FIG. 2C, the number of wirings can be madesmaller.

A light-emitting device in FIG. 3A has the structure in FIG. 2A, and inaddition, includes a wiring 159 and a wiring 160; thus, a connectionrelation of the field-effect transistor 111 and a connection relation ofthe field-effect transistor 117 in FIG. 3A are different from those inFIG. 2A. Further, the field-effect transistor 112 is not provided in thelight-emitting device in FIG. 3A unlike in the light-emitting device inFIG. 2A.

In the light-emitting device in FIG. 3A, the gate of the field-effecttransistor 111 is electrically connected to the wiring 159. The firstgate of the field-effect transistor 113 is electrically connected to theother of the source and the drain of the field-effect transistor 111.The gate of the field-effect transistor 117 is electrically connected tothe wiring 160.

A connection relation of the capacitor 121 in a light-emitting device inFIG. 3B is different from that in the light-emitting device in FIG. 3A.

In the light-emitting device in FIG. 3B, the other of the pair ofelectrodes of the capacitor 121 is electrically connected to the otherof the source and the drain of the field-effect transistor 111.

A connection relation of the field-effect transistor 116 in thelight-emitting device in FIG. 3C is different from that in thelight-emitting device in FIG. 3B. Further, the wiring 156 is notprovided in the light-emitting device in FIG. 3C unlike in thelight-emitting device in FIG. 3B.

In the light-emitting device in FIG. 3C, the one of the source and thedrain of the field-effect transistor 116 is electrically connected tothe first gate of the field-effect transistor 113. The other of thesource and the drain of the field-effect transistor 116 is electricallyconnected to the other of the pair of electrodes of the capacitor 122.Note that the field-effect transistor 113 may be an enhancementtransistor.

With the structure in FIG. 3C, the number of wirings can be madesmaller.

With any of the structures in FIGS. 3A to 3C, the number of field-effecttransistors can be made smaller.

Then, examples of a light-emitting device including a capacitor foradjusting a voltage applied to the light-emitting element 140 will bedescribed with reference to FIGS. 4A to 4C.

A light-emitting device in FIG. 4A has the structure of thelight-emitting device in FIG. 2A, and in addition, includes a capacitor123.

In the light-emitting device in FIG. 4A, one of a pair of electrodes ofthe capacitor 123 is electrically connected to the one of the anode andthe cathode of the light-emitting element 140. A reference potential issupplied to the one of the pair of electrodes of the capacitor 123.

A light-emitting device in FIG. 4B has the structure of thelight-emitting device in FIG. 2B, and in addition, includes thecapacitor 123. A connection relation of the capacitor 123 in thelight-emitting device in FIG. 4B is the same as that in FIG. 4A.

A light-emitting device in FIG. 4C has the structure of thelight-emitting device in FIG. 2C, and in addition, includes thecapacitor 123. A connection relation of the capacitor 123 in thelight-emitting device in FIG. 4C is the same as that in FIG. 4A.

The structure of the light-emitting device is not limited to those inFIGS. 4A to 4C; for example, the light-emitting device in FIG. 3A, FIG.3B, or FIG. 3C may additionally include a capacitor.

Next, an example of a method of driving the light-emitting device inthis embodiment will be described with reference to FIGS. 5A and 5B andFIGS. 6A and 6B.

As an example of a method of driving the light-emitting device in thisembodiment, a method of driving a light-emitting device in FIG. 5A willbe described with reference to a timing chart in FIG. 5B. Thelight-emitting device in FIG. 5A is a light-emitting device having astructure where the light-emitting element 140 is a light-emitting diodeand the field-effect transistors 111 to 118 are n-channel transistors inthe light-emitting device in FIG. 2A. The anode of the light-emittingdiode corresponding to the light-emitting element 140 is electricallyconnected to the other of the pair of electrodes of the capacitor 122.The cathode of the light-emitting diode corresponding to thelight-emitting element 140 is supplied with a potential Vx.

In the example of a method of driving the light-emitting device in FIG.5A, as shown in FIG. 5B, in a period T11, a high-level (also referred toas VH) signal is input through the wiring 153, a low-level (alsoreferred to as VL) signal is input through the wiring 154, and ahigh-level signal is input through the wiring 155. A potential V11 issupplied to the wiring 156, a potential V12 is supplied to the wiring157, and a potential V13 is supplied to the wiring 158. Note that adifference between the potential V11 and the potential V12 is largerthan the threshold voltage of the field-effect transistor 113 (alsoreferred to as Vth113). Further, the potential V12 is lower than thepotential Vx.

At this time, the field-effect transistors 111, 114, 116, 117, and 118are turned on, and the field-effect transistors 112 and 115 are turnedoff.

Electrical connection between the second gate and the drain of thefield-effect transistor 113 is established so that the potential of thesecond gate and that of the drain of the field-effect transistor 113each become the potential V13. Accordingly, in response to the potentialV13, the threshold voltage of the field-effect transistor 113 isnegatively shifted.

A voltage between the first gate and the source of the field-effecttransistor 113 (this voltage is also referred to as Vgs113) becomesV11−V12. The value of V11−V12 is larger than the threshold voltage ofthe field-effect transistor 113 at this time. Accordingly, thefield-effect transistor 113 is turned on.

Then, in a period T12, a data signal is input through the wiring 151, ahigh-level signal is input through the wiring 153, a low-level signal isinput through the wiring 154, and a low-level signal is input throughthe wiring 155. The potential V11 is supplied to the wiring 156, and thepotential V12 is supplied to the wiring 157.

At this time, the field-effect transistors 111, 114, 116, and 117 areturned on, and the field-effect transistors 112, 115, and 118 are turnedoff.

The field-effect transistor 113 remains on. Accordingly, current flowsbetween the source and the drain of the field-effect transistor 113, sothat the potential of the second gate of the field-effect transistor 113is changed. As a result, the threshold voltage of the field-effecttransistor 113 is positively shifted, and the field-effect transistor113 is turned off at the time when the threshold voltage of thefield-effect transistor 113 becomes V11-V12 or higher. In this manner,the data of the threshold voltage of the field-effect transistor 113 canbe obtained.

The potential of the one of the pair of electrodes of the capacitor 121becomes the potential of the data signal (Vsig) input through the wiring151.

Then, in a period T13, a low-level signal is input through the wiring153, a high-level signal is input through the wiring 154, and alow-level signal is input through the wiring 155. The wiring 152 issupplied with a potential Vdd. Note that the value of the potential Vddis larger than the potential V11. In the period T13, a high-level signalis input through the wiring 154 after a low-level signal is inputthrough the wiring 153; however, one embodiment of the present inventionis not limited to this.

At this time, the field-effect transistors 112 and 115 are turned on,and the field-effect transistors 111, 114, 116, 117, and 118 are turnedoff.

The potential of the first gate of the field-effect transistor 113varies depending on the data signal. Accordingly, the field-effecttransistor 113 is turned on, and thus current flows between the sourceand the drain of the field-effect transistor 113.

Since current flows between the anode and the cathode of thelight-emitting diode corresponding to the light-emitting element 140,the light-emitting diode corresponding to the light-emitting element 140emits light.

For example, in the case where the field-effect transistor 113 operatesin a saturation region, the value of current flowing between the sourceand the drain (Ids) of the field-effect transistor 113 depends on thevalue of the data signal input to the first gate, regardless of thethreshold voltage of the field-effect transistor 113. Accordingly, forexample, in the case where Vgs113 is larger than V11-V12, thefield-effect transistor 113 is turned on; thus, current flows betweenthe source and the drain.

Even in the case where the potential of the other of the source and thedrain of the field-effect transistor 113 is changed due to deteriorationof the field-effect transistor 113 or the like, a voltage between thefirst gate and the source of the field-effect transistor 113 can beprevented from being changed because the first gate and the second gateof the field-effect transistor 113 are in a floating state and thecapacitors 121 and 122 are provided.

Note that a mobility correction period may be provided between theperiod T12 and the period T13 and the potential of the second gate ofthe field-effect transistor 113 may be set depending on the mobility ofthe field-effect transistor 113. Accordingly, an influence offluctuation in mobility of the field-effect transistor 113 can beprevented.

The above is the description of an example of a method of driving thelight-emitting device in FIG. 5A.

Note that one or more of the field-effect transistors 111 to 118 in thesemiconductor device in FIG. 5A may be a p-channel transistor.

Next, as an example of a method of driving the light-emitting device inthis embodiment, a method of driving a light-emitting device in FIG. 6Awill be described with reference to a timing chart in FIG. 6B. Thelight-emitting device in FIG. 6A is a light-emitting device having astructure where the light-emitting element 140 is a light-emitting diodeand the field-effect transistors 111 to 118 are n-channel transistors inthe light-emitting device in FIG. 3A. The anode of the light-emittingdiode corresponding to the light-emitting element 140 is electricallyconnected to the other of the pair of electrodes of the capacitor 122.The cathode of the light-emitting diode corresponding to thelight-emitting element 140 is supplied with the potential Vx.

In the example of a method of driving the light-emitting device in FIG.6A, as shown in FIG. 6B, in a period T21, a high-level signal is inputthrough the wiring 153, a low-level signal is input through the wiring154, a high-level signal is input through the wiring 155, a low-levelsignal is input through the wiring 159, and a high-level signal is inputthrough the wiring 160. The potential V11 is supplied to the wiring 156,the potential V12 is supplied to the wiring 157, and the potential V13is supplied to the wiring 158. Note that a difference between thepotential V11 and the potential V12 is larger than the threshold voltageof the field-effect transistor 113. Further, the potential V12 is lowerthan the potential Vx.

At this time, the field-effect transistors 114, 116, 117, and 118 areturned on, and the field-effect transistors 111 and 115 are turned off.

Electrical connection between the second gate and the drain of thefield-effect transistor 113 is established so that the potential of thesecond gate and that of the drain of the field-effect transistor 113each become the potential V13. Accordingly, in response to the potentialV13, the threshold voltage of the field-effect transistor 113 isnegatively shifted.

A voltage between the gate and the source of the field-effect transistor113 becomes V11-V12. The value of V11-V12 is larger than the thresholdvoltage of the field-effect transistor 113 at this time. Accordingly,the field-effect transistor 113 is turned on.

In a period T22, a high-level signal is input through the wiring 153, alow-level signal is input through the wiring 154, a low-level signal isinput through the wiring 155, a low-level signal is input through thewiring 159, and a high-level signal is input through the wiring 160. Thepotential V11 is supplied to the wiring 156, and the potential V12 issupplied to the wiring 157.

At this time, the field-effect transistors 114, 116, and 117 are turnedon, and the field-effect transistors 111, 115, and 118 are turned off.

At this time, the field-effect transistor 113 remains on. Accordingly,current flows between the source and the drain of the field-effecttransistor 113, so that the potential of the second gate of thefield-effect transistor 113 is changed. As a result, the thresholdvoltage of the field-effect transistor 113 is positively shifted, andthe field-effect transistor 113 is turned off at the time when thethreshold voltage of the field-effect transistor 113 becomes V11-V12 orhigher. In this manner, the data of the threshold voltage of thefield-effect transistor 113 can be obtained.

Then, in a period T23, a low-level signal is input through the wiring153, a low-level signal is input through the wiring 154, a low-levelsignal is input through the wiring 155, a high-level signal inputthrough the wiring 159, and a low-level signal is input through thewiring 160. The data signal is input through the wiring 151.

At this time, the field-effect transistor 111 is turned on, and thefield-effect transistors 114, 115, 116, 117, and 118 are turned off.

At this time, the potential of the first gate of the field-effecttransistor 113 varies depending on the potential of the data signal(Vsig).

Then, in a period T24, a low-level signal is input through the wiring153, a high-level signal is input through the wiring 154, a low-levelsignal is input through the wiring 155, a low-level signal input throughthe wiring 159, and a low-level signal input through the wiring 160. Thepotential Vdd is supplied through the wiring 152. Note that the value ofthe potential Vdd is larger than the potential V11.

At this time, the field-effect transistor 115 is turned on, and thefield-effect transistors 111, 114, 116, 117, and 118 are turned off.

Further, the field-effect transistor 113 is turned on, and thus currentflows between the source and the drain of the field-effect transistor113.

Since current flows between the anode and the cathode of thelight-emitting diode corresponding to the light-emitting element 140,the light-emitting diode corresponding to the light-emitting element 140emits light.

For example, in the case where the field-effect transistor 113 operatesin a saturation region, the value of current flowing between the sourceand the drain (Ids) of the field-effect transistor 113 depends on thevalue of the data signal (Vsig) input to the first gate, regardless ofthe threshold voltage of the field-effect transistor 113. Accordingly,for example, in the case where Vgs113 is larger than V11−V12, thefield-effect transistor 113 is turned on; thus, current flows betweenthe source and the drain.

Even in the case where the potential of the other of the source and thedrain of the field-effect transistor 113 is changed due to deteriorationof the field-effect transistor 113 or the like, a voltage between thefirst gate and the source of the field-effect transistor 113 can beprevented from being changed because the first gate and the second gateof the field-effect transistor 113 are in a floating state and thecapacitors 121 and 122 are provided.

Note that a mobility correction period may be provided between theperiod T23 and the period T24 and the potential of the second gate ofthe field-effect transistor 113 may be set depending on the mobility ofthe field-effect transistor 113. Accordingly, an influence offluctuation in mobility of the field-effect transistor 113 can beprevented.

Note that one or more of the field-effect transistors 111 to 118 in thesemiconductor device in FIG. 6A may be a p-channel transistor.

The above is the description of an example of a method of driving thelight-emitting device in FIG. 6A.

As described with reference to FIGS. 5A and 5B and FIGS. 6A and 6B, inan example of the light-emitting device in this embodiment, a periodduring which the data of the threshold voltage is obtained is providedso that the data of the threshold voltage of the field-effect transistoris obtained in advance. Accordingly, the amount of current flowingbetween the source and the drain of the field-effect transistor can bedetermined regardless of the threshold voltage of the field-effecttransistor; thus, an influence of fluctuation in threshold voltage ofthe field-effect transistor can be prevented. Further, an influence ofdeterioration of the field-effect transistor can be prevented.

In an example of the light-emitting device in this embodiment, thefield-effect transistor including the first gate and the second gate isused. With such a structure, even in the case where the field-effecttransistor is a depletion transistor, the data of the threshold voltageof the field-effect transistor can be obtained. The reason of this is asfollows: since the threshold voltage of the field-effect transistor canbe shifted in response to the potential of the second gate, thefield-effect transistor can be off even when the field-effect transistoris an n-channel transistor, the threshold voltage of the field-effecttransistor in the initial state is a negative value and thus thefield-effect transistor is a normally-on transistor, and a voltagebetween the first gate and the source of the field-effect transistor isnot a negative value. Accordingly, the amount of current flowing betweenthe source and the drain of the field-effect transistor can bedetermined regardless of the threshold voltage of the field-effecttransistor; thus, an influence of fluctuation in threshold voltage ofthe field-effect transistor can be prevented.

As described above, in an example of the light-emitting device in thisembodiment, the amount of current flowing between the source and thedrain of the field-effect transistor can be controlled, and thus anoperation defect can be prevented.

Embodiment 3

In this embodiment, a structure example of a light-emitting deviceincluding a driver circuit will be described with reference to FIG. 7 .

A semiconductor device in FIG. 7 includes a first driver circuit 901, asecond driver circuit 902, and a plurality of light-emitting circuits910.

The first driver circuit 901 has a function of controllinglight-emitting operation of the light-emitting circuits 910.

The first driver circuit 901 is formed using a shift register, forexample.

The second driver circuit 902 has a function of controllinglight-emitting operation of the light-emitting circuits 910.

The second driver circuit 902 is formed using a shift register or ananalog switch, for example.

The plurality of light-emitting circuits 910 is arranged in a matrix ina light-emitting portion 900. For the light-emitting circuits 910, thestructure of the light-emitting device in Embodiment 2 can be employed.In this case, a signal is supplied from the first driver circuit 901 tothe wiring electrically connected to the gate of the field-effecttransistor in the light-emitting device in Embodiment 2. Further, a datasignal is supplied from the second driver circuit 902 to the wiring towhich the data signal is input in the light-emitting device inEmbodiment 2.

Note that the first driver circuit 901 may be provided over the samesubstrate as the light-emitting circuits 910.

The above is the description of the structure example of thelight-emitting device in FIG. 7 .

As described with reference to FIG. 7 , in an example of thelight-emitting device in this embodiment, light-emitting operation ofthe light-emitting circuits can be controlled by the first drivercircuit and the second driver circuit.

Embodiment 4

In this embodiment, an example of a field-effect transistor that can beused in the semiconductor device or the light-emitting device in any ofthe above embodiments will be described.

Structure examples of field-effect transistors in this embodiment willbe described with reference to FIGS. 8A and 8B.

A field-effect transistor in FIG. 8A includes, over an element formationlayer 400_A, a conductive layer 401_A, an insulating layer 402_A, asemiconductor layer 403_A, a conductive layer 405 a_A, a conductivelayer 405 b_A, and an insulating layer 406.

A field-effect transistor in FIG. 8B includes, over an element formationlayer 400_B, a conductive layer 401_B, an insulating layer 402_B, asemiconductor layer 403_B including regions 404 a and 404 b, aconductive layer 405 a B, a conductive layer 405 b_B, and an insulatinglayer 407.

Next, the components illustrated in FIGS. 8A and 8B will be described.

As the element formation layers 400_A and 400_B, insulating layers orsubstrates having insulating surfaces can be used, for example.

Each of the conductive layers 401_A and 401_B functions as a gate of thefield-effect transistor. Note that a layer functioning as a gate of thefield-effect transistor can be called gate electrode or gate wiring.

As the conductive layers 401_A and 401_B, it is possible to use, forexample, a layer (single layer or stack of layers) including a metalmaterial such as molybdenum, magnesium, titanium, chromium, tantalum,tungsten, aluminum, copper, neodymium, or scandium or an alloy materialcontaining any of these materials as a main component.

Each of the insulating layers 402_A and 402_B functions as a gateinsulating layer of the field-effect transistor.

Each of the insulating layers 402_A and 402_B can be formed using, forexample, a layer (single layer or stack of layers) including a materialsuch as silicon oxide, silicon nitride, silicon oxynitride, siliconnitride oxide, aluminum oxide, aluminum nitride, aluminum oxynitride,aluminum nitride oxide, hafnium oxide, or lanthanum oxide.

Alternatively, as the insulating layers 402_A and 402_B, an insulatinglayer of a material containing, for example, an element that belongs toGroup 13 in the periodic table and oxygen can be used.

Examples of the material containing a Group 13 element and oxygeninclude gallium oxide, aluminum oxide, aluminum gallium oxide, andgallium aluminum oxide. Note that the amount of aluminum is larger thanthat of gallium in atomic percent in aluminum gallium oxide, whereas theamount of gallium is larger than that of aluminum in atomic percent ingallium aluminum oxide.

Each of the semiconductor layers 403_A and 403_B functions as a layer inwhich a channel of the field-effect transistor is formed (also referredto as channel formation layer), that is, a layer including a channelformation region. For the semiconductor layers 403_A and 403_B, asemiconductor containing an element that belongs to Group 14 in theperiodic table (e.g., silicon) can be used, for example. For example, asemiconductor layer containing silicon may be a single crystalsemiconductor layer, a polycrystalline semiconductor layer, amicrocrystalline semiconductor layer, or an amorphous semiconductorlayer.

For the semiconductor layers 403_A and 403_B, a semiconductor having awider bandgap than silicon, for example, a bandgap of 2 eV or more,preferably 2.5 eV or more, and further preferably 3 eV or more can beused, for example. For example, for the semiconductor layers 403_A and403_B, an oxide semiconductor of metal oxide such as an In-based oxide(e.g., indium oxide), a Sn-based oxide (e.g., tin oxide), or a Zn-basedoxide (e.g., zinc oxide) can be used.

As the metal oxide, a four-component metal oxide, a three-componentmetal oxide, or a two-component metal oxide can also be used, forexample. Note that a metal oxide that can be used as the above oxidesemiconductor may include gallium as a stabilizer for reducing variationin characteristics. A metal oxide that can be used as the above oxidesemiconductor may include tin as the stabilizer. A metal oxide that canbe used as the above oxide semiconductor may include hafnium as thestabilizer. A metal oxide that can be used as the above oxidesemiconductor may include aluminum as the stabilizer. A metal oxide thatcan be used as the above oxide semiconductor may include one or more ofthe following materials as the stabilizer: lanthanum, cerium,praseodymium, neodymium, samarium, europium, gadolinium, terbium,dysprosium, holmium, erbium, thulium, ytterbium, and lutetium, which arelanthanoid. Further, the metal oxide that can be used as the oxidesemiconductor may contain silicon oxide.

Examples of the four-component metal oxide include an In—Sn—Ga—Zn-basedoxide, an In—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, anIn—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn-based oxide, and anIn—Hf—Al—Zn-based oxide.

Examples of the three-component metal oxide include an In—Ga—Zn-basedoxide, an In—Sn—Zn-based oxide, an In—Al—Zn-based oxide, aSn—Ga—Zn-based oxide, an Al—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide,an In—Hf—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-basedoxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, anIn—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide,an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-basedoxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, anIn—Yb—Zn-based oxide, and an In—Lu—Zn-based oxide.

Examples of the two-component metal oxide include an In—Zn-based oxide,a Sn—Zn-based oxide, an Al—Zn-based oxide, a Zn—Mg-based oxide, aSn—Mg-based oxide, an In—Mg-based oxide, an In—Sn-based oxide, and anIn—Ga-based oxide.

As the oxide semiconductor, a material represented by InLO₃(ZnO)_(m) (inis larger than 0) can be used. Here, L in InLO₃(ZnO)_(m) represents oneor more metal elements selected from Ga, Al, Mn, and Co.

For example, as the oxide semiconductor, an In—Ga—Zn-based oxide with anatomic ratio of In:Ga:Zn=1:1:1 (=⅓:⅓:⅓) or In:Ga:Zn=2:2:1 (=⅖:⅖:⅕), orany of oxides whose composition is in the neighborhood of the abovecompositions can be used. Moreover, as the oxide semiconductor, anIn—Sn—Zn-based oxide with an atomic ratio of In:Sn:Zn=1:1:1 (=⅓:⅓:⅓),In:Sn:Zn=2:1:3 (=⅓:⅙:½), or In:Sn:Zn=2:1:5 (=¼:⅛:⅝) or any of oxideswhose composition is in the neighborhood of the above compositions canbe used. For example, a sputtering target with which the semiconductorlayers to be formed have the above composition is preferably used forforming the semiconductor layers.

In the case where the semiconductor layers 403_A and 403_B are formedusing an oxide semiconductor, the semiconductor layers may be in asingle crystal state, a polycrystalline (also referred to aspolycrystal) state, or an amorphous state.

As the semiconductor layers 403_A and 403_B, an oxide semiconductorlayer including a c-axis aligned crystalline oxide semiconductor(CAAC-OS) may be used.

In the CAAC-OS, a mixed phase structure including a crystal region andan amorphous region is formed. Further, in a crystal in the crystalregion, the direction of the c-axis is perpendicular to a surface wherethe semiconductor layer is formed or a surface of the semiconductorlayer, triangular or hexagonal atomic arrangement which is seen from thedirection perpendicular to the a-b plane is formed, and metal atoms arearranged in a layered manner or metal atoms and oxygen atoms arearranged in a layered manner when seen from the direction perpendicularto the c-axis. Therefore, the CAAC-OS is not completely single crystalor completely amorphous. Note that in the case where the CAAC-OS has aplurality of crystal regions, the directions of the a-axis and theb-axis may vary among different crystals of the plurality of crystalregions.

The size of a crystal in the crystal region in the CAAC-OS is estimatedto be about several nanometers to several tens of nanometers. However,in observation of the CAAC-OS with a transmission electron microscope(also referred to as TEM), a boundary between a crystal region and anamorphous region in the CAAC-OS is not necessarily clear. A grainboundary is not found in the CAAC-OS. Thus, since the CAAC-OS includes aregion having no grain boundary, a reduction in electron mobility due tothe grain boundary is less likely to be caused.

In the CAAC-OS, distribution of the crystal regions is not necessarilyuniform. For example, in the case where crystal growth occurs from asurface side of an oxide semiconductor layer to form an oxidesemiconductor layer including CAAC-OS, in some cases, the proportion ofcrystal regions in the vicinity of a surface of the CAAC-OS of the oxidesemiconductor layer is high and the proportion of amorphous regions inthe vicinity of a surface where the CAAC-OS of the oxide semiconductorlayer is formed is high.

Since the c-axes of crystals in the crystal regions in the CAAC-OS areperpendicular to the surface where the CAAC-OS of the oxidesemiconductor layer is formed or the surface of the CAAC-OS of the oxidesemiconductor layer, the directions of the c-axes may be different fromeach other depending on the shape of the CAAC-OS of the oxidesemiconductor layer (the cross-sectional shape of the surface where theCAAC-OS of the oxide semiconductor layer is formed or the surface of theCAAC-OS of the oxide semiconductor layer). Note that the c-axes in thecrystal regions in the CAAC-OS are substantially perpendicular to thesurface where the CAAC-OS of the oxide semiconductor layer is formed orthe surface of the CAAC-OS of the oxide semiconductor layer.

In the CAAC-OS, nitrogen may be substituted for part of oxygen.

It is preferable that the composition of the crystal regions in theCAAC-OS be represented by In_(1+σ)Ga_(1−σ)O₃(ZnO)_(m) (0<σ<1 and M=1 to3), and the composition of the entire CAAC-OS be represented byIn_(P)Ga_(Q)O_(R)(ZnO)_(M) (0<P<2, 0<Q<2, and M=1 to 3).

In the case where an oxide semiconductor layer including a CAAC-OS isused, a layer below and in contact with the oxide semiconductor layer ispreferably flat. For example, the average surface roughness of the layerbelow and in contact with the oxide semiconductor layer including theCAAC-OS is 1 nm or less, preferably 0.3 nm or less. When the flatness ofthe layer below and in contact with the oxide semiconductor layerincluding the CAAC-OS is improved, the mobility can be made higher thanthat of an oxide semiconductor of only an amorphous component. Forexample, the layer below and in contact with the oxide semiconductorlayer including the CAAC-OS can be planarized by one of or both chemicalmechanical polishing (CMP) and plasma treatment. The plasma treatmentincludes treatment of sputtering rare gas ions off a surface andtreatment of etching a surface with the use of an etching gas.

With the use of an oxide semiconductor layer including a CAAC-OS for afield-effect transistor, a change in electrical characteristics of thefield-effect transistor due to irradiation with visible light orultraviolet light can be reduced; thus, the field-effect transistor canhave high reliability.

The regions 404 a and 404 b illustrated in FIG. 8B are doped with adopant and function as a source and a drain of the field-effecttransistor. As the dopant, at least one of elements of Group 13 in theperiodic table (e.g., boron), elements of Group 15 in the periodic table(e.g., one or more of nitrogen, phosphorus, and arsenic), and rare gaselements (e.g., one or more of helium, argon, and xenon) can be used,for example. A region functioning as a source of the field-effecttransistor can be called source region, and a region functioning as adrain of the field-effect transistor can be called drain region.Addition of the dopant to the regions 404 a and 404 b can reduce theresistance between the regions 404 a and 404 b and the conductivelayers.

The conductive layers 405 a A, 405 b_A, 405 a B, and 405 b_B eachfunction as the source or the drain of the field-effect transistor. Notethat a layer functioning as a source of the field-effect transistor canbe called source electrode or source wiring, and a layer functioning asa drain of the field-effect transistor can be called drain electrode ordrain wiring.

The conductive layers 405 a_A, 405 b A, 405 a_B, and 405 b_B can beformed using, for example, a layer (single layer or stack of layers)including a metal material such as aluminum, magnesium, chromium,copper, tantalum, titanium, molybdenum, or tungsten or an alloy materialcontaining any of the above metal materials as a main component.

Alternatively, the conductive layers 405 a A, 405 b A, 405 a B, and 405b_B can be formed using a layer including a conductive metal oxide.Examples of the conductive metal oxide include indium oxide, tin oxide,zinc oxide, indium oxide-tin oxide, and indium oxide-zinc oxide. Notethat silicon oxide may be contained in the conductive metal oxide thatcan be used for the conductive layers 405 a_A, 405 b_A, 405 a B, and 405b_B.

As the insulating layer 406, for example, a layer (single layer or stackof layers) including a material that can be used for the insulatinglayer 402_A can be used.

As the insulating layer 407, for example, a layer (single layer or stackof layers) including a material that can be used for the insulatinglayer 402_A can be used.

In the case where an oxide semiconductor layer is used as thesemiconductor layer 403_A or the semiconductor layer 403_B, for example,dehydration or dehydrogenation is performed; thus, impurities such ashydrogen, water, a hydroxyl group, and a hydride (also referred to ashydrogen compound) are removed from the oxide semiconductor layer, andin addition, oxygen is supplied to the oxide semiconductor layer. Insuch a manner, the oxide semiconductor layer can be highly purified. Forexample, a layer containing oxygen is used as the layer in contact withthe oxide semiconductor layer, and heat treatment is performed; thus,the oxide semiconductor layer can be highly purified.

For example, heat treatment is performed at a temperature higher than orequal to 400° C. and lower than or equal to 750° C., or higher than orequal to 400° C. and lower than the strain point of the substrate. Heattreatment may be further performed in a later step. As a heat treatmentapparatus for the heat treatment, for example, an electric furnace or anapparatus for heating an object by heat conduction or heat radiationfrom a heater such as a resistance heater can be used; for example, arapid thermal anneal (RTA) apparatus such as a gas rapid thermal anneal(GRTA) apparatus or a lamp rapid thermal anneal (LRTA) apparatus can beused. An LRTA apparatus is an apparatus for heating an object to beprocessed by radiation of light (an electromagnetic wave) emitted from alamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, acarbon arc lamp, a high pressure sodium lamp, or a high pressure mercurylamp. A GRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas. As the high-temperature gas, a rare gas or aninert gas (e.g., nitrogen) which does not react with the object by theheat treatment can be used.

Further, after the heat treatment is performed and while the heatingtemperature is being maintained or being decreased, a high-purity oxygengas, a high-purity N₂O gas, or ultra-dry air (having a dew point −40° C.or lower, preferably −60° C. or lower) may be introduced into thefurnace where the heat treatment has been performed. It is preferablethat the oxygen gas or the N₂O gas do not contain water, hydrogen, andthe like. The purity of the oxygen gas or the N₂O gas which isintroduced into the heat treatment apparatus is preferably 6N or higher,further preferably 7N or higher; that is, the impurity concentration inthe oxygen gas or the N₂O gas is preferably 1 ppm or lower, furtherpreferably 0.1 ppm or lower. By the action of the oxygen gas or the N₂Ogas, oxygen is supplied to the oxide semiconductor layer, and defectsdue to oxygen vacancy in the oxide semiconductor layer can be reduced.Note that the high-purity oxygen gas, high-purity N₂O gas, or ultra-dryair may be introduced during the heat treatment.

When an oxide semiconductor layer including a CAAC-OS is formed, theoxide semiconductor film is formed by sputtering while the temperatureof the element formation layer where the oxide semiconductor film isformed ranges from 100° C. to 600° C., preferably from 150° C. to 550°C., more preferably from 200° C. to 500° C. The oxide semiconductor filmis deposited while the temperature of the element formation layer ishigh, whereby the atomic arrangement in the oxide semiconductor film isordered, the density thereof is increased, so that a polycrystal or aCAAC-OS is easily formed. Furthermore, since an oxygen gas atmosphere isemployed for the deposition, an unnecessary atom such as a rare gas atomis not contained in the film, so that a polycrystal or a CAAC-OS iseasily formed. Note that a mixed gas atmosphere including an oxygen gasand a rare gas may be used. In that case, the percentage of an oxygengas is higher than or equal to 30 vol. %, preferably higher than orequal to 50 vol. %, more preferably higher than or equal to 80 vol. %.

With the use of the highly purified oxide semiconductor layer for thefield-effect transistor, the carrier density of the oxide semiconductorlayer can be lower than 1×10¹⁴/cm³, preferably lower than 1×10¹²/cm³,further preferably lower than 1×10¹¹/cm³. The off-state current of thefield-effect transistor per micrometer of channel width can be 10 aA(1×10⁻¹⁷ A) or less, 1 aA (1×10⁻¹⁸ A) or less, 10 zA (1×10⁻²⁰ A) orless, further 1 zA (1×10⁻²¹ A) or less, and furthermore 100 yA (1×10⁻²²A) or less. It is preferable that the off-state current of thefield-effect transistor be as low as possible; the lower limit of theoff-state current of the field-effect transistor in this embodiment isestimated to be about 10⁻³⁰ A/μm.

As described with reference to FIGS. 8A and 8B, a semiconductor deviceor a light-emitting device can be manufactured in such a manner that anexample of the field-effect transistor in this embodiment is used as afield-effect transistor in the semiconductor device or thelight-emitting device in the above embodiment.

Embodiment 5

In this embodiment, a structure example of a light-emitting device willbe described. Note that here, for example, a light-emitting device hasthe circuit configuration in FIG. 2A.

A light-emitting device in this embodiment includes a first substratewhere a semiconductor element such as a field-effect transistor isprovided (the substrate is also referred to as an active matrixsubstrate), a second substrate, and a light-emitting element providedbetween the first substrate and the second substrate.

A structure example of the active matrix substrate in the light-emittingdevice in this embodiment will be described with reference to FIGS. 9Ato 9C. FIGS. 9A to 9C illustrate a structure example of an active matrixsubstrate in the light-emitting device in this embodiment. FIG. 9A is aschematic plan view. FIG. 9B is a schematic cross-sectional view takenalong line A-B in FIG. 9A. FIG. 9C is a schematic cross-sectional viewtaken along line C-D in FIG. 9A. Note that the components illustrated inFIGS. 9A to 9C include those having sizes different from the actualsizes. For convenience, in FIG. 9B, part of cross section taken alongline A-B in FIG. 9A is not shown. Further, in FIG. 9C, part of crosssection taken along line C-D in FIG. 9A is not shown.

The active matrix substrate in FIGS. 9A to 9C includes a substrate 500,conductive layers 511 a to 511 h, an insulating layer 512, semiconductorlayers 513 a to 513 h, conductive layers 515 a to 5151, an insulatinglayer 516, and conductive layers 517 a and 517 b.

The conductive layers 511 a to 511 h are provided on one plane of thesubstrate 500.

The conductive layer 511 a functions as, for example, the gate of thefield-effect transistor 111, the gate of the field-effect transistor114, the gate of the field-effect transistor 116, the gate of thefield-effect transistor 117, and the wiring 153 in the light-emittingdevice in FIG. 2A.

The conductive layer 511 b functions as, for example, the gate of thefield-effect transistor 112, the gate of the field-effect transistor115, and the wiring 154 in the light-emitting device in FIG. 2A.

The conductive layer 511 c functions as, for example, the wiring 156 inthe light-emitting device in FIG. 2A.

The conductive layer 511 d functions as, for example, the first gate ofthe field-effect transistor 113 in the light-emitting device in FIG. 2A.

The conductive layer 511 e functions as, for example, the other of thepair of electrodes of the capacitor 121 and the other of the pair ofelectrodes of the capacitor 122 in the light-emitting device in FIG. 2A.

The conductive layer 511 f functions as, for example, the wiring 157 inthe light-emitting device in FIG. 2A.

The conductive layer 511 g functions as, for example, the gate of thefield-effect transistor 118 and the wiring 155 in the light-emittingdevice in FIG. 2A.

The conductive layer 511 h functions as, for example, the wiring 158 inthe light-emitting device in FIG. 2A.

The insulating layer 512 is provided over the conductive layers 511 a to511 h. The insulating layer 512 functions as, for example, gateinsulating layers of the field-effect transistors 111 to 118 anddielectric layers of the capacitors 121 and 122 in the light-emittingdevice in FIG. 2A.

The semiconductor layer 513 a overlaps with the conductive layer 511 awith the insulating layer 512 therebetween. The semiconductor layer 513a functions as, for example, a channel formation layer of thefield-effect transistor 111 in the light-emitting device in FIG. 2A.

The semiconductor layer 513 b overlaps with the conductive layer 511 bwith the insulating layer 512 therebetween. The semiconductor layer 513b functions as, for example, a channel formation layer of thefield-effect transistor 112 in the light-emitting device in FIG. 2A.

The semiconductor layer 513 c overlaps with the conductive layer 511 awith the insulating layer 512 therebetween. The semiconductor layer 513c functions as, for example, a channel formation layer of thefield-effect transistor 116 in the light-emitting device in FIG. 2A.

The semiconductor layer 513 d overlaps with the conductive layer 511 dwith the insulating layer 512 therebetween. The semiconductor layer 513d functions as, for example, a channel formation layer of thefield-effect transistor 113 in the light-emitting device in FIG. 2A.

The semiconductor layer 513 e overlaps with the conductive layer 511 bwith the insulating layer 512 therebetween. The semiconductor layer 513e functions as, for example, a channel formation layer of thefield-effect transistor 115 in the light-emitting device in FIG. 2A.

The semiconductor layer 513 f overlaps with the conductive layer 511 awith the insulating layer 512 therebetween. The semiconductor layer 513f functions as, for example, a channel formation layer of thefield-effect transistor 117 in the light-emitting device in FIG. 2A.

The semiconductor layer 513 g overlaps with the conductive layer 511 awith the insulating layer 512 therebetween. The semiconductor layer 513g functions as, for example, a channel formation layer of thefield-effect transistor 114 in the light-emitting device in FIG. 2A.

The semiconductor layer 513 h overlaps with the conductive layer 511 gwith the insulating layer 512 therebetween. The semiconductor layer 513h functions as, for example, a channel formation layer of thefield-effect transistor 118 in the light-emitting device in FIG. 2A.

The conductive layer 515 a is electrically connected to thesemiconductor layer 513 a. The conductive layer 515 a functions as, forexample, the one of the source and the drain of the field-effecttransistor 111 and the wiring 151 in the light-emitting device in FIG.2A.

The conductive layer 515 b is electrically connected to thesemiconductor layers 513 a and 513 b. Further, the conductive layer 515b overlaps with the conductive layer 511 e with the insulating layer 512therebetween. The conductive layer 515 b functions as, for example, theother of the source and the drain of the field-effect transistor 111,the one of the source and the drain of the field-effect transistor 112,and the one of the pair of electrodes of the capacitor 121 in thelight-emitting device in FIG. 2A.

The conductive layer 515 c is electrically connected to thesemiconductor layer 513 c. In addition, the conductive layer 515 c iselectrically connected to the conductive layer 511 c through an openingpenetrating the insulating layer 512. The conductive layer 515 cfunctions as, for example, the one of the source and the drain of thefield-effect transistor 116 in the light-emitting device in FIG. 2A.

The conductive layer 515 d is electrically connected to thesemiconductor layer 513 b. The conductive layer 515 d overlaps with thesemiconductor layer 513 c. In addition, the conductive layer 515 d iselectrically connected to the conductive layer 511 d through an openingpenetrating the insulating layer 512. The conductive layer 515 dfunctions as, for example, the other of the source and the drain of thefield-effect transistor 112 and the other of the source and the drain ofthe field-effect transistor 116 in the light-emitting device in FIG. 2A.

The conductive layer 515 e is electrically connected to thesemiconductor layer 513 d, the semiconductor layer 513 e, and thesemiconductor layer 513 g. The conductive layer 515 e functions as, forexample, the one of the source and the drain of the field-effecttransistor 113, the one of the source and the drain of the field-effecttransistor 114, and the other of the source and the drain of thefield-effect transistor 115 in the light-emitting device in FIG. 2A.

The conductive layer 515 f is electrically connected to thesemiconductor layer 513 d. In addition, the conductive layer 515 f iselectrically connected to the conductive layer 511 e through an openingpenetrating the insulating layer 512. The conductive layer 515 ffunctions as, for example, the other of the source and the drain of thefield-effect transistor 113 in the light-emitting device in FIG. 2A.

The conductive layer 515 g is electrically connected to thesemiconductor layer 513 e. The conductive layer 515 g functions as, forexample, the one of the source and the drain of the field-effecttransistor 115 and the wiring 152 in the light-emitting device in FIG.2A.

The conductive layer 515 h is electrically connected to thesemiconductor layer 513 g. The conductive layer 515 h overlaps with theconductive layer 511 e with the insulating layer 512 therebetween. Theconductive layer 515 h functions as, for example, the other of thesource and the drain of the field-effect transistor 114 and the one ofthe pair of electrodes of the capacitor 122 in the light-emitting devicein FIG. 2A.

The conductive layer 515 i is electrically connected to thesemiconductor layer 513 h. In addition, the conductive layer 515 i iselectrically connected to the conductive layer 511 h through an openingpenetrating the insulating layer 512. The conductive layer 515 ifunctions as, for example, the one of the source and the drain of thefield-effect transistor 118 in the light-emitting device in FIG. 2A.

The conductive layer 515 j is electrically connected to thesemiconductor layer 513 h. The conductive layer 515 j functions as, forexample, the other of the source and the drain of the field-effecttransistor 118 in the light-emitting device in FIG. 2A.

The conductive layer 515 k is electrically connected to thesemiconductor layer 513 f. In addition, the conductive layer 515 k iselectrically connected to the conductive layer 511 f through an openingpenetrating the insulating layer 512. The conductive layer 515 kfunctions as, for example, the one of the source and the drain of thefield-effect transistor 117 in the light-emitting device in FIG. 2A.

The conductive layer 515 l is electrically connected to thesemiconductor layer 513 f. In addition, the conductive layer 515 l iselectrically connected to the conductive layer 511 e through an openingpenetrating the insulating layer 512. The conductive layer 515 lfunctions as, for example, the other of the source and the drain of thefield-effect transistor 117 in the light-emitting device in FIG. 2A.

The insulating layer 516 is provided over the semiconductor layers 513 ato 513 h and the conductive layers 515 a to 515 l.

The conductive layer 517 a overlaps with the semiconductor layer 513 dwith the insulating layer 516 therebetween. In addition, the conductivelayer 517 a is electrically connected to the conductive layers 515 h and515 j through openings penetrating the insulating layer 516. Theconductive layer 517 a functions as, for example, the second gate of thefield-effect transistor 113 in the light-emitting device in FIG. 2A.

The conductive layer 517 b is electrically connected to the conductivelayer 515 f through an opening penetrating the insulating layer 516.

Next, a structure example of the light-emitting device in thisembodiment will be described with reference to FIG. 10 . FIG. 10 is aschematic cross-sectional view illustrating a structure example of thelight-emitting device in this embodiment. Note that in this embodiment,a light-emitting element in the light-emitting device emits light towardthe top surface side of the light-emitting device; however, structuresof light-emitting devices according to one embodiment of the presentinvention are not limited thereto. The light-emitting device may emitlight toward the bottom surface side.

The light-emitting device illustrated in FIG. 10 includes an insulatinglayer 518, a conductive layer 519, an insulating layer 521, alight-emitting layer 522, a conductive layer 523, a substrate 524, acoloring layer 525, an insulating layer 526, and an insulating layer 527in addition to the active matrix substrate illustrated in FIGS. 9A to9C.

The insulating layer 518 is provided over the insulating layer 516, theconductive layer 517 a, and the conductive layer 517 b.

The conductive layer 519 is provided over the insulating layer 518. Theconductive layer 519 is electrically connected to the conductive layer517 b through an opening penetrating the insulating layer 518. Theconductive layer 519 functions as, for example, the one of the anode andthe cathode of the light-emitting element 140 in FIG. 2A.

The insulating layer 521 is provided over the conductive layer 519.

The light-emitting layer 522 is electrically connected to the conductivelayer 519 through an opening provided in the insulating layer 521. Thelight-emitting layer 522 functions as, for example, a light-emittinglayer of the light-emitting element 140 in FIG. 2A.

The conductive layer 523 is electrically connected to the light-emittinglayer 522. The conductive layer 523 functions as, for example, the otherof the anode and the cathode of the light-emitting element 140 in FIG.2A.

Note that in an example of the light-emitting device in this embodiment,the light-emitting element has a structure in which light is emittedupwardly; however, one embodiment of the present invention is notlimited thereto, and the light-emitting element may have a structure inwhich light is emitted downwardly.

The coloring layer 525 is provided on one plane of the substrate 524 soas to transmit light with a specific wavelength which is emitted fromthe light-emitting layer 522.

The insulating layer 526 is provided on one plane side of the substrate524 with the coloring layer 525 therebetween.

The insulating layer 527 is provided between the insulating layer 526and the conductive layer 523.

The components of the light-emitting device described with reference toFIGS. 9A to 9C and FIG. 10 are described.

A glass substrate or a plastic substrate, for example, can be used forthe substrates 500 and 524. Note that the substrates 500 and 524 are notnecessarily provided.

The conductive layers 511 a to 511 h can be formed using a layer (singlelayer or stack of layers) including a material applicable to theconductive layer 401_A in FIG. 8A, for example.

The insulating layer 512 can be formed using a layer (single layer orstack of layers) including a material applicable to the insulating layer402_A in FIG. 8A, for example.

The semiconductor layers 513 a to 513 h can be formed using a layerincluding a material applicable to the semiconductor layer 403_A in FIG.8A, for example.

The conductive layers 515 a to 5151 can be formed using a layer (singlelayer or stack of layers) including a material applicable to theconductive layers 405 a_A and 405 b_A in FIG. 8A, for example.

The insulating layer 516 can be formed using a layer (single layer orstack of layers) including a material applicable to the insulating layer406 in FIG. 8A, for example.

The conductive layers 517 a and 517 b can be formed using a layer(single layer or stack of layers) including a material applicable to theconductive layers 511 a to 511 h, for example.

The insulating layer 518 can be formed using a layer (single layer orstack of layers) including a material applicable to the insulating layer512, for example.

The conductive layer 519 can be formed using a layer (single layer orstack of layers) including a material applicable to the conductivelayers 511 a to 511 h, for example.

As the insulating layer 521, an organic insulating layer or an inorganicinsulating layer can be used, for example.

The light-emitting layer 522 is a layer which emits light of a specificcolor. As the light-emitting layer 522, for example, a light-emittinglayer using a light-emitting material which emits light of a specificcolor can be used. The light-emitting layer 522 can also be formed usinga stack of light-emitting layers which emit light of different colors.As the light-emitting material, an electroluminescent material such as afluorescent material or a phosphorescent material can be used.Alternatively, as the light-emitting material, a material containing aplurality of electroluminescent materials may be used. Thelight-emitting layer 522 emitting white light may be formed with a stackof a layer of a fluorescent material emitting blue light, a layer of afirst phosphorescent material emitting orange light, and a layer of asecond phosphorescent material emitting orange light, for example.Alternatively, as the electroluminescent material, an organicelectroluminescent material or an inorganic electroluminescent materialcan be used. Alternatively, the light-emitting layer may be formedusing, for example, in addition to the above-described light-emittinglayer, one or more of the following layers: a hole-injection layer, ahole-transport layer, an electron-transport layer, and anelectron-injection layer.

The conductive layer 523 can be formed using a layer (single layer orstack of layers) including a light-transmitting material selected fromthe materials that can be used for the conductive layers 511 a to 511 h,for example.

The coloring layer 525 can be formed using a layer which contains dye orpigment, for example, and which transmits light with the wavelengthrange of red, light with the wavelength range of green, or light withthe wavelength range of blue. Alternatively, the coloring layer 525 canbe formed using a layer which transmits cyan light, magenta light, oryellow light and which contains dye or pigment. For example, thecoloring layer 525 is formed by a photolithography method, a printingmethod, an inkjet method, an electrodeposition method, anelectrophotographic method, or the like. By using an inkjet method, forexample, the coloring layer can be manufactured at room temperature,manufactured at a low vacuum, or formed over a large substrate. Sincethe coloring layer can be manufactured without a resist mask,manufacturing cost and the number of steps can be reduced.

The insulating layer 526 can be formed using a layer (single layer orstack of layers) including a material applicable to the insulating layer512, for example. Note that the insulating layer 526 is not necessarilyprovided; however, by providing the insulating layer 526, entry of animpurity from the coloring layer 525 to the light-emitting element canbe prevented.

As the insulating layer 527, a layer (single layer or stack of layers)including a material applicable to the insulating layer 512 or a layerincluding a resin material can be used, for example.

As described with reference to FIGS. 9A to 9C and FIG. 10 , an exampleof the light-emitting device in this embodiment includes alight-emitting element emitting light of a specific color, and acoloring layer transmitting light with a specific wavelength which isemitted from the light-emitting element. This structure enables afull-color image to be displayed without forming a plurality oflight-emitting elements emitting light of different colors, therebyfacilitating the manufacturing process and enhancing yield. For example,a light-emitting element can be formed without a metal mask, andtherefore, a manufacturing process can be simple. Further, contrast ofan image can be improved.

Embodiment 6

In this embodiment, examples of an electronic device will be described.

Structure examples of the electronic devices according to thisembodiment will be described with reference to FIGS. 11A to 11C andFIGS. 12A and 12B. FIGS. 11A to 11C and FIGS. 12A and 12B are schematicviews of structure examples of the electronic devices according to thisembodiment.

An electronic device in FIG. 11A is an example of a portable informationterminal. The portable information terminal in FIG. 11A includes ahousing 1001 a and a display portion 1002 a provided in the housing 1001a.

Note that a side surface 1003 a of the housing 1001 a may be providedwith a connection terminal for connecting the portable informationterminal in FIG. 11A to an external device and/or a button used tooperate the portable information terminal.

In the housing 1001 a of the portable information terminal illustratedin FIG. 11A, a CPU, a main memory, an interface with which signals aretransmitted and received between the external device and each of the CPUand the main memory, and an antenna which transmits and receives signalsto/from the external device are provided. Note that in the housing 1001a, one or plural integrated circuits having a specific function may beprovided.

The portable information terminal illustrated in FIG. 11A has a functionof one or more of a telephone set, an e-book reader, a personalcomputer, and a game machine.

An electronic device in FIG. 11B is an example of a stationaryinformation terminal. The stationary information terminal illustrated inFIG. 11B includes a housing 1001 b and a display portion 1002 b providedin the housing 1001 b.

Note that the display portion 1002 b may be provided on a deck portion1008 of the housing 1001 b.

The stationary information terminal illustrated in FIG. 11B includes aCPU, a main memory, and an interface for transmitting and receivingsignals between the external device and each of the CPU and the mainmemory, in the housing 1001 b. Note that in the housing 1001 c, one orplural integrated circuits having a specific function may be provided.Note that the stationary information terminal in FIG. 11B may be furtherprovided with an antenna which transmits and receives signals to/fromthe external device.

Further, a side surface 1003 b of the housing 1001 b in the stationaryinformation terminal in FIG. 11B may be provided with one or more partsselected from a ticket ejection portion that ejects a ticket or thelike, a coin slot, and a bill slot.

The stationary information terminal in FIG. 11B serves, for example, asan automated teller machine, an information communication terminal forticketing or the like (also referred to as a multi-media station), or agame machine.

FIG. 11C illustrates an example of a stationary information terminal.The stationary information terminal illustrated in FIG. 11C includes ahousing 1001 c and a display portion 1002 c provided in the housing 1001c. Note that a support for supporting the housing 1001 c may also beprovided.

Note that a side surface 1003 c of the housing 1001 c may be providedwith a connection terminal for connecting the stationary informationterminal in FIG. 11C to an external device and/or a button used tooperate the stationary information terminal.

The stationary information terminal illustrated in FIG. 11C may includea CPU, a main memory, and an interface for transmitting and receivingsignals between the external device and each of the CPU and the mainmemory, in the housing 1001 c. Note that in the housing 1001 c, one orplural integrated circuits having a specific function may be provided.Note that the stationary information terminal in FIG. 11C may be furtherprovided with an antenna which transmits and receives signals to/fromthe external device.

The stationary information terminal in FIG. 11C serves, for example, asa digital photo frame, an output monitor, or a television set.

The structure of the light-emitting device in the above embodiment canbe used for, for example, a display portion of an electronic device; forexample, the light-emitting device in Embodiment 2 can be used as thedisplay portions 1002 a to 1002 c in FIGS. 11A to 11C.

Further, an electronic device illustrated in FIGS. 12A and 12B is anexample of a folding information terminal. FIG. 12A is a schematicexternal view, and FIG. 12B is a block diagram.

The electronic device in FIGS. 12A and 12B includes a housing 6000 a, ahousing 6000 b, a panel 6001 a, a panel 6001 b, a hinge 6002, a button6003, a connection terminal 6004, and a storage medium insertion portion6005, as illustrated in FIG. 12A. In addition, the electronic device inFIGS. 12A and 12B has a power source portion 6101, a wirelesscommunication portion 6102, an arithmetic portion 6103, an audio portion6104, and a panel portion 6105, as illustrated in FIG. 12B.

The panel 6001 a is provided in the housing 6000 a.

The panel 6001 b is provided in the housing 6000 b. The housing 6000 bis connected to the housing 6000 a with the hinge 6002.

The panel 6001 a and the panel 6001 b function as display panels. Forexample, the panel 6001 a and the panel 6001 b may display differentimages or one image.

As the panel 6001 a and the panel 6001 b, the light-emitting device inEmbodiment 2 can be used.

Further, one of or both the panel 6001 a and the panel 6001 b mayfunction as a touch panel. In this case, data may be input in such amanner that an image of a keyboard is displayed on one of or both thepanel 6001 a and the panel 6001 b and then touched with a finger 6010 orthe like. Alternatively, the display panel and the touch panel may bestacked, so that one of or both the panel 6001 a and the panel 6001 bare formed. Further alternatively, one of or both the panel 6001 a andthe panel 6001 b may be formed with the use of an input-output panelprovided with a display circuit and a light detection circuit.

In the electronic device illustrated in FIGS. 12A and 12B, the housing6000 a can be made to overlap with the housing 6000 b by moving thehousing 6000 a or the housing 6000 b with the use of the hinge 6002, sothat the electronic device can be folded.

The button 6003 is provided on the housing 6000 b. Alternatively, thebutton 6003 may be provided on the housing 6000 a. Furtheralternatively, a plurality of buttons 6003 may be provided on one of orboth the housing 6000 a and the housing 6000 b. For example, when thebutton 6003 which is a power button is provided and pushed, the state ofthe electronic device can be controlled, i.e., the electronic device canbe set to an on state or an off state.

The connection terminal 6004 is provided on the housing 6000 a.Alternatively, the connection terminal 6004 may be provided on thehousing 6000 b. Further alternatively, a plurality of connectionterminals 6004 may be provided on one of or both the housing 6000 a andthe housing 6000 b. For example, when the electronic device is connectedto a personal computer via the connection terminal 6004, data stored inthe electronic device may be rewritten using the personal computer.

The storage medium insertion portion 6005 is provided on the housing6000 a. Alternatively, the storage medium insertion portion 6005 may beprovided on the housing 6000 b. Further alternatively, a plurality ofstorage medium insertion portions 6005 may be provided on one of or boththe housing 6000 a and the housing 6000 b. For example, when a cardstorage medium is inserted into the storage medium insertion portion,data can be read from the card storage medium and written to theelectronic device, or data can be read from the electronic device andwritten to the card storage medium.

The power source portion 6101 has a function of supplying power fordriving the electronic device. For example, from the power sourceportion 6101, power is supplied to the wireless communication portion6102, the arithmetic portion 6103, the audio portion 6104, and the panelportion 6105. The power source portion 6101 is provided with a powerstorage device, for example. The power storage device is provided in oneof or both the housing 6000 a and the housing 6000 b. Note that a powersupply circuit which generates a power supply voltage for driving theelectronic device may be provided in the power source portion 6101. Inthis case, in the power supply circuit, the power supply voltage isgenerated using power supplied from the power storage device. Further,the power source portion 6101 may be connected to a commercial powersupply.

The wireless communication portion 6102 has a function of transmittingand receiving electric waves. For example, the wireless communicationportion 6102 is provided with an antenna, a demodulation circuit, amodulation circuit, and the like. In this case, for example, electricwaves are transmitted and received at the antenna, whereby data isexchanged with an external device. Note that a plurality of antennas maybe provided in the wireless communication portion 6102.

The arithmetic portion 6103 has a function of conducting arithmeticprocessing in response to instruction signals input from the wirelesscommunication portion 6102, the audio portion 6104, and the panelportion 6105, for example. For example, the arithmetic portion 6103 isprovided with a CPU, a logic circuit, a memory circuit, and the like.

The audio portion 6104 has a function of controlling input/output ofsound that is audio data. For example, the audio portion 6104 isprovided with a speaker and a microphone.

The power source portion 6101, the wireless communication portion 6102,the arithmetic portion 6103, and the audio portion 6104 are provided,for example, inside one of or both the housing 6000 a and the housing6000 b.

The panel portion 6105 has a function of controlling operation of thepanel 6001 a (also referred to as panel A) and the panel 6001 b (alsoreferred to as panel B). Note that a driver circuit for controlling thedriving of the panel 6001 a and the panel 6001 b may be provided in thepanel portion 6105 so that operation of the panel 6001 a and the panel6001 b can be controlled.

Note that a control circuit may be provided in one or a plurality of thepower source portion 6101, the wireless communication portion 6102, thearithmetic portion 6103, the audio portion 6104, and the panel portion6105, thereby controlling operation. Further, a control circuit may beprovided in the arithmetic portion 6103, thereby controlling operationof one or a plurality of the power source portion 6101, the wirelesscommunication portion 6102, the audio portion 6104, and the panelportion 6105.

Further, a memory circuit may be provided in one or a plurality of thepower source portion 6101, the wireless communication portion 6102, theaudio portion 6104, and the panel portion 6105, whereby data necessaryfor operation may be stored in the memory circuit. Thus, operation speedcan be improved.

The electronic device illustrated in FIGS. 12A and 12B can receiveelectric power from the commercial power supply and use electric powerstored in the power storage device. Thus, even when electric powercannot be supplied from the commercial power supply because of poweroutage or the like, the electronic device can be operated with the useof the power storage device as a power supply.

When the structure shown in FIGS. 12A and 12B is employed, theelectronic device in FIGS. 12A and 12B can have one or a plurality offunctions of a telephone set, an e-book reader, a personal computer, anda game machine, for example.

The above is the description of an example of an electronic device inthis embodiment.

As described with reference to FIGS. 11A to 11C and FIGS. 12A and 12B,the example of the electronic device in this embodiment has a structurein which the panel portion including the light-emitting device describedin the above embodiment is provided.

In addition, in the examples of electronic devices in this embodiment,the housings may be each provided with one or more of a photoelectricconversion portion which generates power supply voltage according toincident illuminance of light and an operation portion for operating theelectronic device. For example, when the photoelectric conversionportion is provided, an external power supply is not needed; thus, theelectronic device can be used for a long time even in an environmentwhere an external power supply is not provided.

This application is based on Japanese Patent Application serial no.2011-202690 filed with Japan Patent Office on Sep. 16, 2011, the entirecontents of which are hereby incorporated by reference.

1. (canceled)
 2. A light-emitting device comprising a pixel, the pixelcomprising: a first transistor, a second transistor comprising a firstgate electrode and a second gate electrode, a third transistor, a fourthtransistor, a first capacitor, a second capacitor, and a light-emittingelement, wherein the first gate electrode of the second transistorcomprises a region overlapping with the second gate electrode through achannel formation region of the second transistor, wherein one of asource and a drain of the first transistor is electrically connected toa first wiring, wherein the other of the source and the drain of thefirst transistor is electrically connected to one electrode of the firstcapacitor and one of the first gate electrode and the second gateelectrode of the second transistor, wherein one of a source and a drainof the second transistor is electrically connected to the otherelectrode of the first capacitor, one electrode of the second capacitor,one of a source and a drain of the fourth transistor, and thelight-emitting element, wherein the other of the source and the drain ofthe second transistor is electrically connected to one of a source and adrain of the third transistor, wherein the other of the source and thedrain of the third transistor is electrically connected to a secondwiring, wherein the other of the source and the drain of the fourthtransistor is electrically connected to a third wiring, wherein a gateelectrode of the first transistor is electrically connected to a firstgate signal line, wherein a gate electrode of the third transistor iselectrically connected to a second gate signal line, wherein thelight-emitting device comprises: a first conductive layer functioning asthe first wiring; a second conductive layer functioning as the secondwiring; a first semiconductor layer including a channel formation regionof the first transistor; and a second semiconductor layer including achannel formation region of the fourth transistor, and wherein, whenseen from above, each of the first conductive layer, the secondconductive layer, the first semiconductor layer, and the secondsemiconductor layer has a region extending in the same direction.
 3. Alight-emitting device comprising a pixel, the pixel comprising: a firsttransistor, a second transistor comprising a first gate electrode and asecond gate electrode, a third transistor, a fourth transistor, a firstcapacitor, a second capacitor, and a light-emitting element, wherein thefirst gate electrode of the second transistor comprises a regionoverlapping with the second gate electrode through a channel formationregion of the second transistor, wherein one of a source and a drain ofthe first transistor is electrically connected to a first wiring,wherein the other of the source and the drain of the first transistor iselectrically connected to one electrode of the first capacitor and oneof the first gate electrode and the second gate electrode of the secondtransistor, wherein one of a source and a drain of the second transistoris electrically connected to the other electrode of the first capacitor,one electrode of the second capacitor, one of a source and a drain ofthe fourth transistor, and the light-emitting element, wherein the otherof the source and the drain of the second transistor is electricallyconnected to one of a source and a drain of the third transistor,wherein the other of the source and the drain of the third transistor iselectrically connected to a second wiring, wherein the other of thesource and the drain of the fourth transistor is electrically connectedto a third wiring, wherein a gate electrode of the first transistor iselectrically connected to a first gate signal line, and wherein a gateelectrode of the third transistor is electrically connected to a secondgate signal line.
 4. A light-emitting device comprising a pixel, thepixel comprising: a first transistor, a second transistor comprising afirst gate electrode and a second gate electrode, a third transistor, afourth transistor, a first capacitor, a second capacitor, and alight-emitting element, wherein the first gate electrode of the secondtransistor comprises a region overlapping with the second gate electrodethrough a channel formation region of the second transistor, wherein oneof a source and a drain of the first transistor is electricallyconnected to a first wiring, wherein the other of the source and thedrain of the first transistor is electrically connected to one electrodeof the first capacitor and one of the first gate electrode and thesecond gate electrode of the second transistor, wherein one of a sourceand a drain of the second transistor is electrically connected to theother electrode of the first capacitor, one electrode of the secondcapacitor, one of a source and a drain of the fourth transistor, and thelight-emitting element, and wherein the other of the source and thedrain of the second transistor is electrically connected to one of asource and a drain of the third transistor.